Conventionally, among various image forming apparatuses such as copy machines and laser beam printers, a both-surface image forming apparatus in which image formation is automatically performed on both surfaces, that is, a front surface and a back surface of a sheet-like recording medium composed of paper or the like is known. In such a both-surface image forming apparatus, first of all, in an image formation unit having an image supporter such as a photoconductive drum, a toner image is formed on a first surface (front surface) of a recording medium, and the toner image is subjected to first heating treatment by a fixing means so as to fix the image on the recording medium. Then, the recording medium on which the toner image is fixed on the first surface (front surface) thereof is sent to a both-surface transporting means and reversed upside-down by a switchback operation or the like. The reversed recording medium is fed again to the image formation unit, and a toner image is formed on a second surface (back surface) of the recording medium. The toner image is subjected to second heating treatment by the fixing means and fixed on the recording medium. In this manner, image formation on both the front and back surfaces of the recording medium is automatically performed.
However, when image formation is performed on both front and back surfaces of a recording medium in the above described manner, a curl (rolling tendency) is tend to be generated in the recording medium when first fixation heating treatment at a high temperature with respect to the first surface (front surface) of the recording medium is performed, and, due to the curl (rolling tendency), sometimes the recording medium winds around the image supporter such as a photoconductive drum and is not readily separated upon image formation with respect to the second surface (back surface) of the recording medium. More specifically, first of all, there is a tendency that the first surface (front surface), which is the heated side of the recording medium, largely thermally expands toward the second surface (back surface) due to the heating treatment upon fixation with respect to the toner on the first surface (front surface) of the recording medium; therefore, a curl having a shape in which the entirety of the recording medium after fixation is warped like a curvature toward the second surface (back surface) side is generated. Then, when the recording medium, which is curled toward the second surface (back surface) side, is fed again to the image formation unit and image formation is performed, the curl-generated part of the recording medium winds around the outer peripheral surface of the image supporter such as the photoconductive drum. As a result, there is a possibility that the recording medium, which normally exits the image formation unit while it is separated from the image supporter, cannot be transported, thereby leading to a jammed state.
With respect to such a problem of transporting failure due to generation of the curl upon image formation on both the front and back surfaces of the recording medium, conventionally, a countermeasure of uniformly lowering the heating treatment temperature (fixation temperature adjustment) upon fixation on the first surface (front surface) of the recording medium, thereby reducing the generation amount of curls has been sometimes employed. However, in this case, fixation performance across the entirety of the image formation region of the first surface (front surface) may be significantly deteriorated.
There is also a proposal in which the heating treatment temperature (fixation temperature adjustment) merely with respect to a trailing end region of the first surface (front surface) of the recording medium in the transporting direction is lowered, thereby suppressing the curl generation amount in the region of the second surface (back surface) that serves as a leading end in the transporting direction and preventing deterioration in the fixation performance in the entirety of the image formation region (for example, see Japanese Patent Application Laid-Open (kokai) No. 1998-10915). However, this proposal also has a problem that deterioration in the fixation performance with respect to the region in which the heating treatment temperature upon fixation is lowered, that is, the transporting-direction trailing end region of the first surface (front surface) cannot be prevented.
Therefore, it is an object of the present invention to provide an image forming apparatus capable of simultaneously preventing both defective transportation and defective fixation due to the curl generating phenomenon upon both-surface image formation of the recording medium.